In an emission tomography system, photons from the inside of a body, or in case of system tune-up, from a phantom and the like that is to be imaged are detected. These photons pass from the interior of the body to a detector outside the body and may be attenuated by different parts of the body, depending on the composition of the body. Generally, the emission tomography may be a Positron Emission Tomography (PET), a Single Photon Emission Computed Tomography (SPECT), and the like. In order to reconstruct a three-dimensional distribution of a radionuclide within the body or phantom accurately, this attenuation must be considered.
In a combined MR (magnetic resonance) and emission tomography imaging system, a subject may not be imaged entirely in a MR image due to technical limitations of the system and thus, the subject is clipped in the MR image. Therefore, the attenuation imposed by the body of the subject may not be considered accurately while reconstructing the emission tomography image, and thus, the emission tomography image may not be accurate.